FIG. 1 shows the cross section of a conventional lateral DMOS (LDMOS) transistor. In the formation of conventional LDMOS transistors, for instance, N-type semiconductor substrate 10 is provided. A field oxide layer 11 is formed on the substrate in a floating region, then a gate 12 is formed thereon. P-type ions are implanted to form a P-type base region 13 using the gate 12 as a mask. Then, the P-type base region 13 is driven-in, so that the P-type base region 13 extends under the gate 12. N-type ions are implanted in the P-type base region 13 using the gate as a mask and then driven-in to form an N-type source region 14.
in the above discussed LDMOS transistor, the channel length of the LDMOS transistor 1 is determined by the drive-in steps, which can not be accurately controlled in order to obtain a short channel. But a short channel is critical for better characteristics of the transistor.
The cross-section of a conventional vertical DMOS (VDMOS) transistor during various steps in its formation is shown in FIGS. 2a to 2c. As shown in FIG. 2a, oxide sidewall spacers 22 are formed on the sidewalls of a gate 21 on an N-type substrate 20. Thereafter, as shown in FIG. 2b, P-type ions are implanted in the N-type substrate 20 to form P-type base regions 23. Referring to FIG. 2c, the sidewall spacers 22 are then removed. N-type ions are implanted in the P-type base regions and driven-in, to form N-type source region 24.
However, the above mentioned method for fabricating VDMOS transistors can not be applied to LDMOS transistor fabrication for reducing its channel length, because the field oxide layer in the floating region will be etched when removing the oxide sidewall spacers.